Electromagnetic chucks are used on various machine tools such as, for example, milling machines, drill presses, lathes, and surface grinders for holding a workpiece in position while a machining operation is effected upon the workpiece. The chucks hold the workpiece by means of a magnetic field. Traditional electromagnetic chucks comprise one or a plurality of electrical coils inducing magnetic flux lines in electromagnetic cores made of a ferromagnetic material such as low carbon, high permeable steel or cast iron. The electric coils are wound in such direction and the holding surfaces of the diverse electromagnets are arranged such that areas of opposite magnetic polarities are engaged by the workpiece, with the result that the workpiece, also made of ferromagnetic material, is held on the surface of the electromagnetic cores. Operation of a traditional electromagnetic chuck requires application of a continuous direct current to maintain the magnetic flux through the coils. A typical controller for such an electromagnetic chuck can be seen in Littwin, U.S. Pat. No. 3,579,053.
One of the drawbacks of utilizing a traditional electromagnetic chuck is its tendency to retain residual magnetism, called hysteresis, after turning off the current inducing the magnetization. It has been found that to demagnetize the chuck, and the engaged workpiece, a sequence of successive reversals of current in the electrical windings of the chuck with successively decreasing current can be applied until the current (and residual magnetism) approaches zero. Because of the high inductance present in the electromagnetic chuck coils and workpiece, this process can require excessive expenditure of energy and time.
Another disadvantage of traditional electromagnetic chucks is their requirement that a continuous current be applied through the chuck to maintain the magnetic field, and, hence, the holding power of the chuck. If power to the chuck is lost for any reason, the chuck will lose some or all of its magnetism. This is particularly a danger if a workpiece is being subjected to surface grinding at the time the power is lost.
To overcome some of the drawbacks of traditional electromagnetic chucks, through experimentation with different composite magnetic materials, certain manufacturers, including Eclipse and Tecnomagnete, have developed electric permanent chucks that dispense with the need to continuously apply a direct current to maintain a magnetic flux through the coils of the chuck. Unlike traditional electromagnetic chucks, which are generally made of steel or cast iron, these permanent chucks are manufactured from various rare earth metals and are somewhat more expensive than traditional electromagnetic chucks. The nature of the materials used to manufacture the chucks allows the device to be magnetized in only one direction, i.e., the device has only one direction of polarization.
Accordingly, to energize fully an electric permanent chuck, a direct current of sufficient power need only be supplied to the chuck coils for a short time in the polarizing direction, on the order of tenths of seconds. Even if power is lost to the controller after the chuck is magnetized, the electric permanent chuck will remain magnetized and continue to hold the workpiece. To demagnetize the chuck, a demagnetization charge is applied to the chuck in a direction opposite to the direction of the original charge on the chuck. Because of the natural direction of the polarity of the chuck, the chuck can generally be demagnetized in a single step, that is, the charge can be reduced to zero by applying a current in the direction opposite to that utilized to magnetize the chuck in a single step and for a relatively short period of time. Once this is done, there is little to no residual magnetism present in the chuck. This is in contrast to traditional electromagnetic chucks that require a series of reduced, reversing charges to demagnetize fully the chuck and the workpiece.
Even though the magnetic charge of the electric permanent chuck can be reduced to zero in a single step, because the workpiece (which may be made of steel) may still hold a residual charge, it is often desirable (for instance in grinding operations where the operator may want to manually remove a workpiece for testing) to remove any residual magnetism in the workpiece. Thus, the present invention incorporates a method of demagnetizing electromagnetic chucks by applying a series of oppositely directed and successively decreasing current to the chuck coils in a novel way also to remove residual charge in the workpiece being held by the electric permanent chuck.
An example of a controller for an electric permanent chuck is shown in U.S. Pat. No. 5,267,119 to Armond et al. This controller, unlike the present invention, requires the use of three-phase AC for applying a greater voltage than the source voltage to the chuck to demagnetize the chuck. Moreover, this controller does not contain circuitry for applying a series of reverse charges to the chuck to demagnetize a workpiece. The present invention is adapted to work with a single AC voltage source, does not contain any power transformers or capacitors between the source of power and the chuck to be magnetized and demagnetized, and combines circuitry for removing residual magnetization of a workpiece. The present invention also provides a plurality of outputs for use with multiple, independently controlled chucks and with multiple coils on a single chuck, including circuitry for simultaneously applying current in opposite directions to adjacent coils of the electric permanent chuck.